Just said this to someone else earlier today. They didn’t need the interior armrests when it was a bench, they don’t need them now. I did the same thing to my 2017 earlier this week with baby #2 due soon.
Make sure you get this weathertech (#4410402), or huskyliner (#14371). The weathertech piece covers all of the second and third row. I found it to be far superior to the husky, which had gaps in a few places, and you would need the second row husky mat, if you don’t have anything there, presently.
You’ll also need to seal the hole in the floor which the center seat covers with a grommet of sorts.
Overall, it’s a refreshing change, and makes the truck appear incredibly more spacious inside.